Applications sometimes need to establish and manage a session between computing devices. A session is a set of interactions between computing devices that occurs over a period of time. As an example, real-time communications applications such as MICROSOFT WINDOWS MESSENGER or Voice over Internet Protocol (“VoIP”) establish sessions between communicating devices on behalf of a user. These applications may use various mechanisms to establish sessions, such as a “Session Initiation Protocol” (“SIP”). SIP is an application-layer control protocol that devices can use to discover one another and to establish, modify, and terminate sessions between devices. SIP is an Internet proposed standard. Its specification, “RFC 3261,” is available at <http://www.ietf.org/rfc/rfc3261.txt>. A specification for extensions to SIP relating to event notifications, “RFC 3265,” is available at <http://www.ietf.org/rfc/rfc3265.txt>. Both of these specifications are incorporated herein in their entirety by reference.
A SIP network comprises entities that can participate in a dialog as a client, server, or both. SIP supports four types of entities: user agent, proxy server, redirect server, and registrar. User agents initiate and terminate sessions by exchanging messages with other SIP entities. A user agent can be a user agent client, which is generally a device that initiates SIP requests, or a user agent server, which is a device that generally receives SIP requests and responds to such requests. As examples, “IP-telephones,” personal digital assistants, and any other type of computing device may be user agents. A device can be a user agent client in one dialog and a user agent server in another, or may change roles during the dialog. A proxy server is an entity that acts as a server to clients and a client to servers. In so doing, proxy servers intercept, interpret, or forward messages between clients and servers. A redirect server accepts a SIP request and generates a response directing the client that sent the request to contact an alternate network resource. A registrar is a server that accepts registration information from SIP clients and informs a location service of the received registration information.
SIP supports two message types: requests, which are sent from a client to a server, and responses, which are sent from a server to a client, generally when responding to a request. A SIP message is comprised of three parts. The first part of a SIP message is a “start line,” which includes fields to indicate a message type and a protocol version. The second part of a SIP message comprises header fields whose values are represented as name-value pairs. The third part of a SIP message is the message's body, which is used to describe the session to be initiated or contain data that relates to the session. Message bodies may appear in requests or responses.
SIP messages are routed based on the contents of their header fields. To be valid, a SIP request should contain at least the following six header fields: To, From, CSeq, Call-ID, Max-Forwards, and Via. The To header field indicates the logical identity of the recipient of the request. The From header field indicates the logical identity of the initiator of the request. The Max-Forwards header field indicates the number of hops a request can make before arriving at its destination. As an example, if a message from device A transits device B before arriving at destination device C, the message is said to have made two hops (e.g., to devices B and C). The Via header field indicates the path taken by the request so far (e.g., a sequence of network addresses of devices through which the request has transited) and indicates the path that should be followed when routing the response. Various network devices may insert Record-Route header fields when forwarding a SIP message in an attempt to force subsequent messages in a dialog to be routed through the device. The Record-Route header field may contain an identifier (e.g., network address) for the device and parameters. Devices that handle a message may force the message to be routed to devices listed in a message's Route header field. The Route header field values may be based on the Record-Route header field values inserted by devices. These and other header fields are described in the SIP specification referenced above.
A common form of real-time conversation is provided by instant messaging services. An instant messaging service allows participants at end points to send messages and have them received within a second or two by the other participants in the conversation. The receiving participants can then send responsive messages to the other participants in a similar manner. To be effective, a real-time conversation relies on the participants' becoming aware of, reviewing, and responding to received messages very quickly. This quick response is in contrast to conventional electronic mail systems in which the recipients of electronic mail messages respond to messages at their convenience.
When an initiating participant wants to start a real-time conversation, that participant needs to know whether the intended participants are available to respond in real time to a message. If not, then communications via conventional electronic mail, voice mail, or some other mechanism may be more appropriate. For example, if the computers of the intended participants are currently powered off, then a real-time conversation may not be possible. Moreover, if their computers are currently powered on, but the intended participants are away from their computers, a real-time conversation is also not possible. The initiating participant would like to know the availability of the intended participants so that an appropriate decision on the form of communication can be made.
The availability status of an entity such as a computer system (i.e., end point) or a user associated with that computer system is referred to as “presence information.” Presence information identifies the current “presence state” of the user. Users make their presence information available so that other users can decide how best to communicate with them. For example, the presence information may indicate whether a user is logged on (“online”) with an instant messaging server or is logged off (“offline”). Presence information may also provide more detailed information about the availability of the user. For example, even though a user is online, that user may be away from their computer in a meeting. In such a case, the presence state may indicate “online” and “in a meeting.”
In an instant messaging context, a publishing user (“publisher”) may provide their presence information to a presence server that then provides the presence information to subscribing users (“subscribers”). Thus, a presence server may use a subscriber/publisher model to provide the presence information for the users of the presence service. Whenever the presence information of a user changes, the presence server is notified of the change by that user's computer system and in turn notifies the subscribing users of the change. A subscribing user can then decide whether to initiate an instant messaging conversation based on the presence information of the intended participants. For example, if the presence information indicates that a publishing user is currently in a conference telephone call, then the subscribing user may decide to send an instant message, rather than place a telephone call, to the publishing user. If the subscribing user, however, needs to call and speak with the publishing user, the subscribing user needs to monitor the presence information of the publishing user to know when the call can be placed. When the subscribing user notices that the publishing user's presence information indicates that the telephone conference has been concluded, the subscribing user can then place the telephone call.
FIG. 1 is a block diagram illustrating communications between a client computer system and a presence server. The client computer system 130 subscribes to presence information of a user that is provided to presence server 120 by that user's computer system. The client computer system and the presence server are considered to be end points of the communications. The client computer system communicates with the presence server via communications link 140 and a server pool 110. The server pool includes a load balancer 111, servers 112, and a server database 113. The server pool may provide SIP registration services and proxy services. The client computer system and the presence server register their location with the SIP registration service of the server pool using a SIP registration request. When the client computer system wants to subscribe to the presence information of a particular contact, the client computer system sends a SIP SUBSCRIBE request to the presence server. When the load balancer of the server pool receives the SIP SUBSCRIBE request, it selects a server to route the request through as an intermediary. The selected server may access the server database to identify the location of the presence server and forward the SIP SUBSCRIBE request to the presence server. The presence server then responds with a SIP 200 OK response assuming that it accepts the subscription. As the SIP SUBSCRIBE request travels on its way to the presence server, the route is recorded in the SIP header. All subsequent communications during that session travel through the recorded route. When the presence information of the contact changes, the presence server sends a notification message to the client computer system that travels through the same route.
A difficulty occurs when the intermediary server of the server pool fails. After a failure occurs, the presence server will continue to send notifications of changes in presence information to the client computer system via the failed intermediary server. Because the intermediary server has failed, the notifications will not be forwarded to the client computer system. As a result, the client computer system's presence information will become out of date or unsynchronized with the actual presence information of the contact stored in the presence server. It would be desirable to have a technique that would allow for presence information or other state information associated with a client and server to be re-synchronized when it is determined that the information may be unsynchronized.